ENH: add optional agglomeration coefficent to random decomposition

- specifies the number of consecutive cells to assign to the same
  randomly chosen processor. Can be used to have a less extremely
  random distribution for testing possible breaking points.

Eg,
    method random;

    coeffs
    {
        agglom  4;
    }

- Add finiteArea cellID (actually face ids) / faceLabel and procID
  for foamToVTK with -write-ids. Useful when this type of information
  is needed.

applications/utilities/postProcessing/dataConversion/foamToVTK/convertAreaFields.H

@@ -49,8 +49,10 @@ if (doFiniteArea)
         faMeshPtr = faMesh::TryNew(meshProxy.baseMesh());
     }
 
-    if (faMeshPtr && nAreaFields)
+    if (faMeshPtr && (nAreaFields || withMeshIds))
     {
+        const faMesh& areaMesh = faMeshPtr();
+
         reportFields::area(Info, objects);
 
         const auto& pp = faMeshPtr->patch();
@@ -66,17 +68,36 @@ if (doFiniteArea)
             Pstream::parRun()
         );
 
-        writer.beginFile(faMeshPtr->name());
+        writer.beginFile(areaMesh.name());
 
         writer.writeTimeValue(timeValue);
         writer.writeGeometry();
 
-        writer.beginCellData(nAreaFields);
+        // Optionally with (cellID, faceLabels, procID) fields
+        writer.beginCellData
+        (
+            (withMeshIds ? 2 + (writer.parallel() ? 1 : 0) : 0)
+          + nAreaFields
+        );
+
+        if (withMeshIds)
+        {
+            const globalIndex procAddr(areaMesh.nFaces());
+
+            // Use global indexed values for the 'cell' ids
+            writer.writeCellData("cellID", identity(procAddr.range()));
+
+            // Use proc-local data for faceLabels
+            // (confusing enough already without renumbering)
+            writer.writeCellData("faceLabels", areaMesh.faceLabels());
+
+            writer.writeProcIDs();  // parallel only
+        }
 
         writeAllAreaFields
         (
             writer,
-            *faMeshPtr,
+            areaMesh,
             objects,
             true // syncPar
         );

src/parallel/decompose/decompositionMethods/randomDecomp/randomDecomp.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019-2021 OpenCFD Ltd.
+    Copyright (C) 2019-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -51,9 +51,29 @@ Foam::labelList Foam::randomDecomp::randomMap(const label nCells) const
 
     labelList finalDecomp(nCells);
 
-    for (label& val : finalDecomp)
+    if (agglom_ > 1)
     {
-        val = rndGen.position<label>(0, nDomains_ - 1);
+        label cached = 0;
+        label repeat = 0;
+
+        for (label& val : finalDecomp)
+        {
+            if (!repeat)
+            {
+                cached = rndGen.position<label>(0, nDomains_ - 1);
+                repeat = agglom_;
+            }
+            --repeat;
+
+            val = cached;
+        }
+    }
+    else
+    {
+        for (label& val : finalDecomp)
+        {
+            val = rndGen.position<label>(0, nDomains_ - 1);
+        }
     }
 
     return finalDecomp;
@@ -65,11 +85,18 @@ Foam::labelList Foam::randomDecomp::randomMap(const label nCells) const
 Foam::randomDecomp::randomDecomp
 (
     const dictionary& decompDict,
-    const word& regionName
+    const word& regionName,
+    int select
 )
 :
-    decompositionMethod(decompDict, regionName)
+    decompositionMethod(decompDict, regionName),
-{}
+    agglom_(1)
+{
+    const dictionary& coeffs = findCoeffsDict(typeName + "Coeffs", select);
+
+    // No sanity check needed here (done in randomMap routine)
+    coeffs.readIfPresent("agglom", agglom_);
+}
 
 
 // * * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * //

src/parallel/decompose/decompositionMethods/randomDecomp/randomDecomp.H

@@ -27,15 +27,25 @@ Class
     Foam::randomDecomp
 
 Description
-    Decomposition according to pseudo-random number generator.
+    Decomposition according to pseudo-random number generator,
+    which is useful for development and stress testing implementations
+    but obviously sub-optimal for normal uses.
+    Can optionally specify the number of consecutive cells to assign
+    to the same processor (agglomeration).
+
+    Coefficients:
+    \table
+        Property  | Description                             | Required | Default
+        agglom    | Number of cells to agglomerate          | no  | 1
+    \endtable
 
 SourceFiles
     randomDecomp.C
 
 \*---------------------------------------------------------------------------*/
 
-#ifndef randomDecomp_H
+#ifndef Foam_randomDecomp_H
-#define randomDecomp_H
+#define Foam_randomDecomp_H
 
 #include "decompositionMethod.H"
 
@@ -50,6 +60,12 @@ class randomDecomp
 :
     public decompositionMethod
 {
+    // Private Data
+
+        //- Number of cells to agglomerate per random value (default: 1)
+        label agglom_;
+
+
     // Private Member Functions
 
         //- Random distribution on the 0-nCells interval
@@ -74,7 +90,8 @@ public:
         explicit randomDecomp
         (
             const dictionary& decompDict,
-            const word& regionName = ""
+            const word& regionName = "",
+            int select = selectionType::DEFAULT
         );